The present invention is directed to an improved method for producing cellulose actetate powder, particularly extrusion-grade cellulose acetate powder, the improved method involving producing the powder in the form of small coagulate particles directly from precipitation, and to the product produced by the method.
For purposes of this disclosure, by "extrusion-grade cellulose acetate powder", it is meant that after the addition of a liquid plasticizer, the powder is characterized by being dry and free-flowing, and is of a suitable tapped bulk density so that it flows uniformly through feed tubes and feed hoppers to an extruder, without becoming caked or without sticking together in clumps or without bridging over a feed hopper opening and thus stopping further flow into an extruder.
In the prior art, cellulose acetate powder in general has been produced by different methods, some methods including grinding cellulose acetate flake into powder; precipitating directly into a powder; however, such powder is usually not suitable for use an extrusion-grade powder because it cakes after liquid plasticizer is added; and precipitating into pellets and then the pellets in a subsequent step are milled to a powder, which may then be suitable for use as an extrusion-grade powder.
An example of a direct powder precipitation process is disclosed in U.S. Pat. No. 2,891,946 (issued June 23, 1959). This process, however, is generally not considered suitable for producing extrusion-grade powder because after the addition of a liquid plasticizer, the cellulose acetate powder then tends not to have free-flowing characteristics.
An example of a product resulting from precipitation of cellulose acetate into pellets and then milling the pellets into powder is the one identified as EASTMAN Cellulose Acetate E-400-25X, a product of Eastman Chemical Products, Inc., a subsidiary of Eastman Kodak Company. The latter product has an average acetyl content (as disclosed in Eastman Technical Data Sheet TDS No. E-131 "EASTMAN Cellulose Acetate for Extrusion Compounding") of about 39.9%, a falling ball viscosity range (as determined by ASTM Method D-1343 in the solution described as Formula A, ASTM Method D-871) of about 17 to about 35 seconds, and a tapped bulk density of about 33 pounds per cubic foot.
The powder particles of the product, EASTMAN Cellulose Acetate E-400-25X, will take up plasticizer up to about 50 percent by weight of the powder particles. For instance, some industrial concerns may blend 52 parts plasticizer with one hundred parts cellulose acetate powder.
The laterally described milled powder particle has a pore volume of about 0.52 to about 0.69 cubic centimeters per gram, and a surface area of about 11 to about 31 square meters per gram, as measured by an independent laboratory, the Materials Analysis Laboratory of Micromeritics Instrument Corporation in Norcross, Georgia.
Whereas the powder particles of the preceding described product are the result of a milling or grinding step, the powder product of the present invention comprises small coagulate particles, generally of a size that will pass through a 45 mesh screen. For instance, some samples of the coagulate powder particles have been found to have a pore volume of about 2.15 cubic centimeters per gram, and a surface area of about 37.4 square meters per gram, as measured by the aforementioned independent laboratory, the Materials Analysis Laboratory of Micromeritics Instrument Corporation in Norcross, Georgia.
The surface area of the coagulate powder particles is substantially porous and is essentially free of fused or smooth surfaces so as to more readily take up plasticizer. When the coagulate powder particles are sliced open to reveal the interior surface, it, too, is found to be substantially porous and essentially free of fused or smooth surfaces.
The coagulate powder particles of the disclosed invention also will take up plasticizer up to about 50 percent by weight of the coagulate powder particles, without the powder particles sticking or clumping and thereby preventing uniform flow of the powder particles to an extruder.
An object of the invention, therefore, is to provide an improved method for producing extrusion-grade cellulose acetate powder by direct precipitation, thus eliminating the need for a subsequent milling or grinding step.
Another object is to provide an improved extrusion-grade cellulose acetate powder resulting from the improved method, the powder being formed of small coagulate particles having a large pore volume and surface area.
Other objects inherent in the nature of the method and product will become evident from this disclosure to those skilled in this art.